Presbyopia is a state where accommodative power of the eyes is deteriorated due to aging and focusing on a near object is difficult. In order to improve this symptom of presbyopia, progressive addition lenses have been developed as spectacle lenses for presbyopia. The progressive addition lenses include, depending on usage (distance range where clear vision is desired), common types of bifocal use, indoor types of occupational use, etc.
For example, a bifocal progressive addition lens includes a region for distance vision (distance portion) at an upper portion of the lens, a region for near vision (near portion) at a lower portion of the lens, and a corridor connecting the distance portion and the near portion having different powers. In the corridor, the power successively varies from a power of the distance portion to a power of the near portion. Thus, even when a line of sight shifts between distance vision and near vision, looking through the corridor allows the line of sight to shift with less discomfort over a wide range from distance vision to near vision.
When designing such a progressive addition lens, first, a reference line for determining a configuration surface of the lens (corridor surface) called a main line of sight or a meridian is determined and then a surface configuring the lens is optimized. Therefore, the main line of sight or the like determines performance of the lens and thus how to determine the main line of sight or the like is quite important. The main line of sight is generally defined as a line along points on the lens where a frequency of a line of sight to pass is the highest when the lens is worn as spectacles and the line of sight shifts from distance vision to near vision (when the line of sight shifts from the upper portion to the lower portion of the lens). Meanwhile, the meridian is defined as a line where arbitrary cross sections of respective points on the line have the same curvature. The meridian is usually arranged at a position where a frequency of a line of sight to pass is the largest. Therefore, when the main line of sight or the like is determined, it is necessary to calculate the position on the lens where a line of sight passes most frequently. A general main line of sight on the lens is illustrated in FIG. 1.
In FIG. 1, an upper portion of a round lens 1′ is positioned at an upper portion of the eye of a wearer when the lens 1′ is worn while a lower portion of the lens 1′ is positioned at a lower portion of the eye of the wearer with a left side of the lens 1′ positioned on a nose side of the wearer and a right side of the lens 1′ positioned on an ear side of the wearer. Therefore, although not illustrated, the upper portion of the lens 1′ corresponds to the distance portion while the lower portion of the lens 1′ corresponds to the near portion with the center and the vicinity thereof of the lens 1′ corresponding to the corridor.
Furthermore, the lens is formed with a hidden mark which cannot be easily recognized by visual inspection. Based on the hidden mark, a predetermined reference point (distance reference point F, near reference point N, fitting point E, etc.) in the lens is determined. In FIG. 1, two hidden marks H are formed on a horizontal line passing through the center of the lens 1′ and a midpoint therebetween corresponds to the center of the lens 1′. Therefore in FIG. 1, a group of points having an equivalent distance from the hidden marks H corresponds to a vertical line (Y axis) passing through the center of the lens 1′. Furthermore, a main line of sight 2′ passes through the vicinity of the distance reference point F and the fitting point E from the upper portion to the vicinity of the center of the lens 1′ in such a manner as to correspond to the vertical line, thereafter is displaced toward the nose side (inclined inward), and extends to the lower portion of the lens 1′ while passing through the vicinity of the near reference point N.
The reason for the main line of sight 2′ to be displaced toward the nose side is because convergence of the eye that occurs with near vision is considered. When an object existing nearby is looked at with the both eyes, the left and right eyeballs are inclined toward the nose side (inner side) as compared to when an object existing far away is looked at and thus the line of sight is also inclined toward the nose side. This is called convergence of the eye. As illustrated in FIG. 1, therefore, the main line of sight 2′ is displaced toward the nose side in the portion lower than the center of the lens 1′ that is mainly used for looking at an object existing nearby.
As a method to determine a main line of sight, for example, Patent Literature 1 describes about determining a meridian considering convergence of the eye that occurs with near vision. In Patent Literature 1, however, neither a prescribed power nor an object distance is considered.
Meanwhile, Patent Literature 2 describes about determining a main line of sight with a focus on that a convergence amount of the eye that occurs with near vision is different depending on a prescribe power; however, an object distance is not considered.
Patent Literature 3 describes about determining a main line of sight considering prescription information such as a power, an addition power, prism, and a pupillary distance (PD) and an object distance.